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Monday, January 08, 2018

Best Chance For Precipitation In Many Weeks

A strong, and quite cold, short wave at 500 mb will move toward and then across Arizona during the next 48 hours (above is 6 UTC WRF-GFS forecast from Atmo, valid at 12 UTC on Wednesday morning the 10th). The coldest air is forecast to remain to our north; however, a substantial vorticity maxima may track nearly overhead.The GEFS plumes for QPF at Tucson (also from 6 UTC) are shown below. The QPF forecasts from the ensemble system have become fairly tight, clustered around amounts of about 2/10 inch.The 6 UTC WRF-GFS QPF forecast (second below valid through midnight on Wednesday night) forecasts 3/10 inch at the airport, with higher amounts to the north and on the Catalinas.

Two complicating factors may impact the way this event evolves. First, the 13 UTC MIMIC analysis of TPW this morning (above) indicates a very substantial plume of high, subtropical moisture has been pulled northward by the old system that has lingered at low latitudes. The interplay between this plume of high PW and the stronger system to the north-northwest will be important for us. The current WRF-GFS forecast for PW (below valid at 6:00 pm tomorrow evening) indicates values of PW a bit over an inch along the front to our west. The model however rapidly weakens the moisture band. The PW field will be important to monitor as the event develops.The second factor of interest is whether or not showers develop well ahead of the strong cold front that will come by tomorrow night - if they do, then amounts could be higher than current model forecasts. Finally, a real weather system to watch.